Rotary machines and in particular electric machines, are well known in the art. In the case of electric machines (both generators and motors), the electric machine includes a stator and a rotor separated by an air gap. The rotor is positioned within the stator and is rotatably mounted on the casing of the electric machine via bearings. For optimum performance of the electric machine, it is desired that the air gap between the rotor and the stator be constant about the entire circumference of the rotor. This ensures a constant magnetic flux between the rotor and stator during operation of the electric machine.
When the shape of both the rotor and stator are exact, if the rotor is positioned within the stator so that its physical, central longitudinal axis is coincident with the physical, central longitudinal axis of the stator, the air gap between the rotor and stator will be constant. Unfortunately, during the manufacturing process, it is ram for the shape of both the rotor and stator to be exact. This deviation in shape between the rotor and stator is especially pronounced in larger equipment. When such a deviation in shape exists, assembly of the electric machine to achieve optimum performance is not a simple task. This is in view of the fact that if the rotor is positioned within the stator with its physical, central longitudinal axis coincident with that of the stator, the air gap between the rotor and stator will not be constant thereby reducing the performance of the electric machine. In cases such as this, it is necessary for the position of the rotor relative to the stator to be adjusted to try to create a constant air gap between the rotor and stator. Thus, the physical, central longitudinal axis of the rotor will not be coincident with that of the stator.
In almost all instances, due to deviations in the physical shape of the rotor and stator, it is not possible to position the rotor so that a constant air gap between the rotor and stator exists. In these instances, it is desired that the rotor be located relative to the stator at the "best position"i.e. a position where the air gap between the rotor and stator is as close to constant as is possible. This allows the performance of the electric machine to be optimized within the constraints imposed by the physical shape of the rotor and stator.
In the past, the best position for the rotor within the stator has been determined manually. Specifically, when the rotor is positioned within the stator, the best position for the rotor is determined by measuring the air gap between a pole of the rotor and a section of the stator manually with feeler gauges. Measurements are taken at both ends of the rotor pole and are recorded. The rotor is then rotated by one pole face and the air gap between the ends of the next rotor pole and the same section of the stator is measured and recorded. This process is continued until the rotor has been rotated through one complete rotation. Once this has been done, the recorded measurements are averaged and plotted manually on a polar chart to determine the best position for the rotor within the stator. The position of the rotor or stator is then adjusted accordingly.
Unfortunately, this method of determining the best position for the rotor suffers from disadvantages. Specifically, because the rotor may include more than 56 poles, this method is extremely tedious, highly labour intensive and time consuming, often requiring up to 36 hours or more per electric machine. Also, in the case of large electric machines, due to the mass of the rotor, it is difficult to stop and start the rotation of the rotor so that it only rotates one pole face at a time. Further, when the measurements are being taken with the feeler gauges, the individuals taking the measurements must extend an arm between the rotor and stator and adjust the feeler gauge to contact the rotor and stator. Practically, this is difficult to do, often making the measured readings from the feeler gauges inaccurate.
Methods of dynamically monitoring the clearance between rotating components have been considered. For example, U.S. Pat. No. 3,628,136 to Jonas and U.S. Pat. No. 4,063,167 to Duly both show an apparatus permanently installed in a turbine for measuring the clearance between the shroud and the tips of the turbine blades. The apparatus includes an electric circuit connected to the shroud which measures the capacitance between itself and the tips of the turbine blades as the blades pass by the electric circuit. Since the magnitude of the measured capacitance depends upon the relative spacing between the electric circuit and the tips of the turbine blades, the clearance between the shroud and turbine blades can be determined by observing the output of the electric circuit. Thus, situations where the turbine blades are spaced too far from the shroud, which may result in back flow through the turbine, can be detected.
Although, the Jonas and Duly patents teach the electrical measurement of the clearance between a shroud and turbine blades, neither reference discloses a method for determining the best position for a rotor within a stator to optimize the performance of a rotary machine.
It is therefore an object of the present invention to provide a novel apparatus and method of determining the best position for an inner member within an outer member in a rotary machine.